Recently I listened to a presentation by DHH from September 2008 on what "legacy" code means. His argument is that legacy isn't an attribute of the code, it's an attribute of the person looking at the code. Further, what you're working on right now is legacy code to someone in the future. Depending on your knowledge of the platform that someone could be you two weeks from now. Or it could be your new team member three years from now.
A couple other things worth noting from the presentation:
1. I thought very interesting his assertion that the better you get at what you're doing, the longer it takes for you to recognize your code as legacy. This isn't because you have blinders on, just that you've internalized a sufficient amount of the platform, patterns and idioms that you're not changing as rapidly, that you're doing more things right the first time. It also means that to get the same 'jumps' in learning you'll be working on projects for a longer time and getting into unsexy 'maintenance', which is underappreciated as a learning area IMO.
2. His honesty at looking back at his own code and reasoning. It can be tough to say that things you've written stink -- and even tougher to do so in front of an audience! -- but IMO it's a necessary part of growing. Part of this is removing intent from the equation: you have to trust that decisions you made at the time were appropriate given your constraints (budget, time, technology), problemspace, and your understanding of all these. Intervening time can change everything, so don't beat yourself up for doing stupid things. Recognize it and move on.
In a recent presentation Steve Vinoski referred to "legacy software" as "software that works". And, to carry forward a point above, legacy software codifies the decisions made based on the organizational knowledge and pressures existing at the time it was planned and written. You might not agree with the decisions, but you need to respect them, because they represent the best efforts of your colleagues and peers. And they're not as stupid as you think.
It's easy for us to look at a system and point out all its flaws. (I've been embarrassingly guilty of this in the past.) But without knowing why things were done you're in danger of making the same mistakes, just using a different technology or perhaps at a different architecture layer.
For instance, how our system represents the nursing assistant plan of care differs radically from that found in the previous version. The plan of care is fairly large (~500 or so items) and when we translated each item from the earlier version we discussed its clinical intent, its effects elsewhere in the system (print reports, documentation to the government, etc.), and whether it was doing its job as intended. We tossed some items that were too specific (e.g., tied to a single customer's process) or unused, added some that weren't covered, and adapted others to be more flexible. But we only did so after at least trying to understand what the original was doing. It took a long time, but it had the beneficial side-effect of getting us familiar with the domain as well as the types of decisions made in it.
For all these changes the external representation to the Nurse is pretty similar. Nurses have a pretty static notion of what such a plan of care looks like and should include, and how it's broken down into sections ('Bathing', 'Meals', etc.). After reviewing all the old items in the plan of care we created a set of subsections ('Cautions', 'Monitoring', 'Special Equipment', etc.) to make sense of the hodgepodge found in the earlier version, and then applied them uniformly to all the different sections. So we made them learn something new, but they generally only needed to learn it once. And people using the system for the first time would have a simple way to locate something on a care plan. We made these decisions with a domain expert and, for the most part, they've held up.
My last idea for now about legacy software is a general irritation. I'd guess that most people writing software today are maintaining or adapting existing systems. Creating new software is sexy and gets lots of press, but that's not where most people are working.
But it seems that most software frameworks are optimized for those creating new systems. I can understand the need to have your 'quick start' tutorial create a new system and start using it because of space constraints -- you need it to be quick! But frameworks rarely discuss what to do when you want to use it with an existing system -- no, I don't want your framework to create my schemas; no, I can't use an artificial primary key because my table already has its own; no, I can't intuit the relationships from table names because I actually have foreign keys in my system.
The main reason projects don't do this is likely that existing domains are too complicated to deal with in documentation -- plus, who likes writing documentation? And I'm sympathetic to that. But I think frameworks are missing a great opportunity to lower the barrier for most of the software development population who might be interested in a framework but have trouble getting started because of the emphasis on greenfield systems.
 Since getting the G1 (and exercising a few times a week) I've been listening to a lot more podcasts, and one of the feeds I'm catching up with is IT Conversations. So it's taking a while to get up to 'current' -- it might not be new to you, but it is to me.
 ...and your knowledge of the domain, which is something I want to write more about.
 I could be wrong about this. I've been heads-down for quite a long time, so my ideas may be out of date. Like everything else about me.